so this is consistently happening. if you have Windows10 with Dropbox 112.4.321 on and syncing and you try openin an .xlsx file from within your dropbox files, then there's about 4-5 seconds delay when opening excel.
If you open excel first, this doesn't happen.
If you have another file opened and you open another one, it's instant.
Already removed all add-ins included Dropbox for Microsoft Office, which i don't use anyway
If you shut down dropbox, it's instant however it's handy having the immediate backup.
You can of course work on your files, do what you have to do and then turn on dropbox so you're synced.